Coatings are typically applied to fabrics to increase water resistance, water transport, insulative ability or heat storage properties of the fabrics. Recently, microencapsulated phase change materials have been described as a suitable component for fabric coatings when exceptional heat transfer and storage capabilities are desired. In particular, International Patent Application No. PCT/US93/05119 for "Fabric with Reversible Enhanced Thermal Properties" to Colvin, et al., which is incorporated herein by reference, discloses that fabrics coated with a binder containing microcapsules filled with energy absorbing phase change material enables the fabric to exhibit extended or enhanced heat retention or storage properties.
Research has demonstrated that applying a binder containing microspheres of phase change materials with commercial coating equipment can be problematic. For example, use of solvent based gravure printing techniques in which a solvent system was employed to achieve uniform dispersion of the microspheres in a binder proved unsuccessful because the solvent systems damaged the microspheres.
Thermoplastic gravure printing techniques also proved unsatisfactory for use with microspheres of phase change material. When using higher temperature thermoplastic gravure printing techniques, sustained temperature of 325.degree. F. caused severe damage to the microspheres. Although lower temperature thermoplastic gravure printing techniques avoided significant damage to the microspheres, the resulting coating was found lacking in washability and durability. Moreover, lower temperature thermoplastic gravure printing techniques precluded addition of the desired amounts of the microspheres, allowing addition of microspheres of up to only about 20% by dry weight of the microsphere/binder material. This low percentage of phase change material in the coating makes the coating susceptible to undesirable heat transfer across the coating, especially in locations where phase change material is sparsely applied.
Attempts to encapsulate microspheres of phase change materials in a thermoplastic spray have also proved unsatisfactory. In particular, scattering microspheres into a stream of sprayed, fibrous thermoplastic material resulted in a binder matrix that did not fully encase the microspheres. The resulting binder/microsphere material was susceptible to loss of microspheres, which worked loose and were continually shedded from the fabric. In addition, the coating lacked uniformity of thickness and microsphere distribution.
Attempts were also made to utilize thermoplastic extrusion techniques to create a film of continuous web in which microspheres of phase change material were uniformly distributed. However, thermal breakdown of the microspheres resulted from the higher temperatures utilized. The extrusion screw employed with these techniques also physically damaged the microspheres.
Phase change materials in microencapsulated form are commonly supplied as a dry powder. This powder is difficult to wet and uniformly disperse in aqueous systems. Moreover, some microencapsulated phase change materials have an internal layer of modified gelatin which is hydrophilic and capable of absorbing its own weight in water. Not only does the hydrophilic quality of such microcapsules make more standard component proportions inapplicable, microcapsules which have absorbed water tend to swell and associate, increasing the viscosity of the coating system above acceptable limits. Although the precise behavior of microcapsules in the coating system which have absorbed water is uncertain, it is believed that such microcapsules agglomerate, reducing their dispersion throughout the binder of the coating system, which de-stabilizes the binder. This de-stabilization can increase over time. When latex binders are used with microencapsulated phase change material, de-stabilization of the latex binder can continue until the latex binder coagulates.
U.S. Pat. Nos. 5,254,380, 5,211,949, 5,282,994 and 5,106,520 for "Dry Powder Mixes Comprising Phase Change Materials" describe free flowing, conformable powder-like mixes of silica particles and a phase change material which the silica particles of between 7.times.10.sup.-3 to 7.times.10.sup.-2 microns are mixed with phase change material in a ratio of up to 80% by weight of phase change material. While these patents describe a matrix in which microspheres of phase change materials need not be separately encapsulated, they do not describe the use of dry powder mixes containing phase change materials in binder matrices for coating fabrics.
It is against this background that the significant improvements and advancement of the present invention have taken place in the field of fabric coatings containing energy absorbing, temperature stabilizing phase change materials and methods of manufacturing same.